Chemiexcitation and melanin in photoreceptor disc turnover and prevention of macular degeneration.

Abstract

Age-related macular degeneration, Stargardt disease, and their <i>Abca4^-/-</i> mouse model are characterized by accelerated accumulation of the pigment lipofuscin, derived from photoreceptor disc turnover in the retinal pigment epithelium (RPE); lipofuscin accumulation and retinal degeneration both occur earlier in albino mice. Intravitreal injection of superoxide (O₂^•-) generators reverses lipofuscin accumulation and rescues retinal pathology, but neither the target nor mechanism is known. Here we show that RPE contains thin multi-lamellar membranes (TLMs) resembling photoreceptor discs, which associate with melanolipofuscin granules in pigmented mice but in albinos are 10-fold more abundant and reside in vacuoles. Genetically over-expressing tyrosinase in albinos generates melanosomes and decreases TLM-related lipofuscin. Intravitreal injection of generators of O₂^•- or nitric oxide (^•NO) decreases TLM-related lipofuscin in melanolipofuscin granules of pigmented mice by ~50% in 2 d, but not in albinos. Prompted by evidence that O₂^•- plus ^•NO creates a dioxetane on melanin that excites its electrons to a high-energy state (termed "chemiexcitation"), we show that exciting electrons directly using a synthetic dioxetane reverses TLM-related lipofuscin even in albinos; quenching the excited-electron energy blocks this reversal. Melanin chemiexcitation assists in safe photoreceptor disc turnover.